LT3040 datasheet(21/30 Pages) AD

21 / 30 page

LT3040

Rev. 0

For more information www.analog.com

voltage. Increasing the VIN cap value will smooth the VIN

pin start-up.

If fast start-up capability is not used, tie FS to VIN.

Figure 10.

3040 TA10

V+

RFAULT

COUT

–

VIN

GND ILIM

EN/UV

V+

VIN

FAULT

RIN

RILIM

CIN

CV+

LT3040

VOUT

RLOAD

VFB

QPWR

OUT

QDRV

Low Pass Filter with Fast-Startup Enabled

VIN Slew Rate

The LT3040’s error amplifier is bandwidth limited. If the

VIN pin slews too fast in the positive direction, the error

amplifier reacts and enters current limit. If the VIN pin

voltage stops slewing, the LT3040’s bandwidth limitation

causes the output to overshoot significantly before set-

tling into regulation. In order to avoid undesirable over-

shoots at the output, ADI recommends limiting the VIN

pin slew rate to less than or equal to 1V/µs.

A similar behavior is expected for negative slew rates on

the VIN pin where the output undershoots significantly. If

VIN slews fast in the negative direction, the error ampli-

fier shuts off and the output discharge is a function of

the LT3040’s load current. Thus, larger undershoots are

expected at heavier load conditions.

To limit slew rates when using the fast start function-

ality of the LT3040, the minimum CIN requirement is

governed by:

100mA • COUT/ILIM ≤ CIN

where ILIM is the output current limit. The internal current

limit for LT3040 is around 280mA (180mA in foldback).

When using a minimum output capacitor without any

external current limit, use at least 2.2µF capacitor at the

VIN pin if using the fast start circuitry. This capacitor limits

the slew rate on the VIN pin while also ensuring that the

VIN pin doesn’t overshoot during fast start up. For appli-

cations with lower current limit or higher output capaci-

tance, a larger VIN capacitor is required.

Power Supply Sequencing

The LT3040 is able to withstand voltages as high as the

Absolute Maximum rating on the FS and VIN pins even

when the part is turned off. However, for optimum applica-

tion performance and to protect the peripheral circuitry

used with the LT3040, ADI recommends that the LT3040

is powered up (or enabled) before or in sync with the volt-

age application at VIN. During turn-off, ADI recommends

powering down VIN before or in sync with the LT3040.

The LT3040’s VIN pin is internally clamped to the VFB pin

(typical clamp voltage of 1.5V), if VIN is powered while

the LT3040 is shutdown, there is a possibility that the

Absolute Maximum rating for the voltage differential from

VIN to VFB or the current rating (max 20mA) on the VIN

and VFB pins or both may be exceeded; damaging the part.

In applications where the VIN supply is not current limited

to less than or equal to 20mA, connect a protection resis-

tor from the Voltage Source (Voltage Reference/Voltage

Output DAC) to the FS/VIN pin. Use a minimum resistor

of 50Ω/V to limit the current below the Abs Max Rating

of 20mA for the VIN to VFB clamp. For example, if con-

necting a 5V reference use at least a 250Ω (5V • 50Ω/V)

resistor. If the VIN pin is powered high while LT3040 is in

shutdown, the DC current flowing through the internal VIN

to VFB clamp may pull output above ground.

Filtering High Frequency Spikes

For applications where the LT3040 is used to post-regu-

late a switching converter, its high PSRR effectively sup-

presses any “noise” present at the switcher’s switching

frequency — typically 100kHz to 4MHz. However, the very

high frequency (100s of MHz) “spikes” — beyond the

LT3040’s bandwidth — associated with the switcher’s

power switch transition times will almost directly pass

APPLICATIONS INFORMATION

LT3040 Datasheet(PDF) 21 Page - Analog Devices

LT3040 Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL